This application discloses an invention that is related, generally and in various embodiments, to a system and method for parallel control of variable frequency drives.
Variable frequency drives are known in the art. A variable frequency drive typically includes a control circuit and a power circuit. For synchronous motor applications, the variable frequency drive may also include a field supply. The control circuit controls the operation of the power circuit and, for synchronous motor applications, also enables/disables the associated field supply. The power circuit includes a rectifier and an inverter, and provides power to a winding of a motor connected to the variable frequency drive. For synchronous motor applications, the field supply provides power to an exciter for a motor field circuit.
The control circuit typically includes a speed regulator, a flux regulator, a magnetizing current regulator, a torque current regulator, a DQ-3Φ transform, a pulse width modulator, and a motor model. The speed regulator provides a magnetizing current reference, and the flux regulator provides a torque current reference. The control circuit compares the magnetizing current reference to a measured magnetizing current, and the magnetizing current regulator determines a Q-axis voltage reference. The control circuit also compares the torque current reference to a measured torque current, and the torque current regulator determines a D-axis voltage reference. Additional feed-forward signals may be added to the D-axis voltage reference and the Q-axis voltage reference to provide a higher dynamic response. The DQ-3Φ transform transforms the Q-axis voltage reference and the D-axis voltage reference from two-phase information into three-phase values. The pulse width modulator converts the three-phase values to switching commands that are sent to the power circuit. The motor model generally utilizes measured voltage and/or current signals to determine motor parameters such as the motor speed, the motor flux, the motor flux angle. For applications where low cost is a business requirement, the motor model may only utilize the variable frequency drive output current or the motor current to determine motor parameters. The motor model also converts measured currents into a magnetizing current component and a torque current component for use in the magnetizing current regulator and the torque current regulator, respectively.
Many of the functions performed by the control circuit are implemented in software. The software is written such that calculations are done at two or more different rates so as to save processor execution time. In general, the pulse width modulator operates at the fastest rate and is usually implemented in hardware. The magnetizing current regulator, the torque current regulator, and the DQ-3Φ transform blocks are typically executed at a data rate of 1-10 kilohertz so that a fast response of the control is achieved in limiting the output current of the variable frequency drive in case of sudden changes in the load or the output circuit. The speed regulator and the flux regulator typically operate at a slower rate of 100-1000 hertz because both motor speed and motor flux change at a much slower rate than the magnetizing current and the torque current. The motor model also is usually computed at this rate. Communications from the control circuit to the outside world, which includes communications to an external device (from the customer), is typically at a rate of 1-10 hertz.
When the power needed to be delivered to a load exceeds the power available from a single variable frequency drive, it is known to connect a number of variable frequency drives in parallel in order to meet the power requirement of the load. To control such parallel arrangements, a master-slave arrangement is often utilized. In a master-slave configuration, a master controller often operates as an interface between external controls (e.g., customer controls) and the various variable frequency drives. The respective control circuits communicate with the master controller and also communicate directly with each other.
In general, for the master-slave arrangement, one variable frequency drive is configured as the master drive, and the other variable frequency drives are configured as slave drives. The master drive typically performs the speed regulator function and the flux regulator function, and issues current commands (torque current reference and magnetizing current reference) to itself and all the slave drives. The slave drives operate on current commands from the master drive and do not utilize their respective speed regulators or flux regulators. Thus, the slave drives operate with fewer regulators than the master drive. The slave drives utilize the current commands from the master drive to regulate each of their output currents and produce the required motor voltage. In addition to the current commands, the master drive must also send either the motor flux angle or the motor frequency information to the slave drives to enable the slave drives to convert the respective Q-axis voltage references and D-axis reference voltages to the correct 3-phase reference frame. To provide good dynamic performance, the required communication rate between the master drive and the slave drives is in the range of 100-1000 hertz. The master controller is required to have knowledge of the master drive so that a slave drive can be configured as the new master drive in case the original master drive needs to be removed from the system.
For proper operation, each variable frequency drive is required to know its position in the system (either master or slave) and is required to operate differently based on its position. As the master drive performs more tasks than the slave drives, the control of the master drive is more complex than the control of the slave drives. Although the master-slave configuration works well for a number of different applications, the master-slave configuration tends to be relatively complex and relatively expensive to implement.